Constantinos Simitis (Greek: Κωνσταντίνος Σημίτης) (born 23 June 1936), usually referred to as Costas Simitis or Kostas Simitis, was Prime Minister of Greece and leader of the Panhellenic Socialist Movement (PASOK) from 1996 to 2004.

Costas Simitis was born in Piraeus to Georgios Simitis, a Professor at the School of Economic and Commercial Sciences, and to his wife Fani (née Christopoulou). He studied Law at the University of Marburg in Germany and economics at the London School of Economics. He is married to Daphne Arkadiou and has two daughters, Fiona and Marilena. His brother Spiros Simitis is a prominent jurist specializing on data privacy in Germany. He currently resides in the Kolonaki district of Athens.

In 1965 he returned to Greece and was one of the founders of the "Alexandros Papanastasiou" political research group . In 1967, after the military coup of 21 April, this group was transformed into Democratic Defense, an organization opposed to the military regime. Simitis escaped abroad after planting bombs in the streets of Athens (in later years he acknowledged his activities on Greek MEGA TV channel) in order to avoid being jailed and became a member of the Panhellenic Liberation Movement (PAK), led by Andreas Papandreou. He also took up a position as university lecturer in Germany. He returned to Athens in 1974 and was one of the co-founders of PAK's successor, the PASOK. In 1977 he took up a lecturer's post at the Panteion University.

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